This invention generally pertains to an injection molding apparatus. More specifically, the present invention relates to a plastic injection molding nozzle which has a fluid assist mechanism.
The invention is particularly applicable to a gas assisted injection molding process in which a nozzle is utilized to inject a viscous fluid, such as a molten plastic, into a mold cavity together with a non-viscous fluid, such as a gas. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid, such as a gas or liquid, are injected into a mold cavity.
Recently, gas assisted injection molding has gained popularity. In this process, the mold cavity is filled with a plastisized thermoplastic material to a volume less than 100% of the mold space and an inert gas is injected under pressure into the plastisized material to fill the rest of the volume in the mold cavity. The gas is injected into the center of the flow of plastic but does not mix with the melt and instead runs along specially designed channels. In this way, with a suitably designed part, a continuous network of hollowed out sections can be provided. The material displaced by the gas from the middle of the sections moves out to fill the remainder of the mold space.
This network of gas channels provides a uniform pressure distribution system throughout the mold space during part rehardening and cool down thus minimizing internal stresses. Gas injection provides a solution to a number of problems that have long plagued the injection molding industry. These include reducing stress and warpage of the plastic part, elimination of sink marks and the provision of smooth surfaces on the injection molded part. In addition, clamp tonnage requirements can be reduced in comparison to conventional injection molding processes. The process also permits different wall thicknesses and faster cycles in comparison with the conventional injection molding processes. Also, gas assisted injection molding reduces the need for external flow runners.
Several types of nozzles are known for gas assisted injection molding. However, many of these nozzles do not vent the gas back through the nozzle when the discharge of the gas is required. Even those nozzles which do vent the gas back through the nozzle are unsatisfactory because the molten plastic remaining in the nozzle or in the sprue and runner system is frequently vented back along with the gas thus causing one of the major difficulties with gas assisted injection molding, namely, that the gas channels in the nozzle become plugged with thermoplastic which solidifies and blocks off further gas flow through these channels. In addition, the gas piping and valves downstream from the nozzle can become plugged. The nozzle then becomes unuseable until it is cleaned out which is a time consuming, difficult and expensive process.
Accordingly, it has been considered desirable to develop a new and improved injection molding nozzle which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.